This invention relates to altimetry systems for aircraft and more particularly to encoding of altimetry data.
All aircraft carry an altimeter to sense aircraft altitude. In most modern aircraft, such equipment includes an encoding altimeter; a barometric altimeter coupled to an altimeter digitizer; and/or an air data computer. The altimetry data output from such devices is digitally encoded for use by other aircraft systems, including, for example: the aircraft transponder; traffic collision and avoidance system (TCAS); ground proximity warning systems (GPWS and EGPWS); flight data recorders; various navigation systems; and in some cases a cockpit display of pressure altitude.
Digital representation of the altimetry data is electronically accomplished through the use of Gray code. Unlike other number systems, in which more than one bit may change when the measured parameter increments in value, the Gray code is a number system designed such that only one bit changes at a time. For example, in the system of counting numbers, a change in value from 79 to 80 changes both the xe2x80x9c7xe2x80x9d to an xe2x80x9c8xe2x80x9d and the xe2x80x9c9xe2x80x9d to a xe2x80x9c0.xe2x80x9d Simultaneously changing the value of more than one bit increases the probability of an error in the encoding process. Such errors are reduced through the use of the Gray code. A particular form of Gray code, called Gillham code, is used in aviation to encode the altimetry data. Table 1 lists the Gillham code.
Errors in altimetry can also occur due to a blocked static port. Barometric altimeters sense changes in ambient air pressure and correlate these pressure changes to changes in altitude. Blockage of the static port disrupts the altimeter""s source of pressure data, thereby introducing altimetry errors. Additional errors can also arise when the altitude sensing equipment has a fault that causes the data, or a bit in the digital representation of the data, to freeze at a certain value. Errors may also occur due to an altimeter malfunction that results in a completely erroneous determination of altitude.
Any errors in the altitude data used by aircraft systems may lead to hazardous conditions or catastrophic accidents. Air traffic controllers often maintain aircraft separation through use of vertical spacing. Without accurate altitude data as relayed by the aircraft transponder, the controller cannot be assured that aircraft are separated vertically. The TCAS system is a safety device that also seeks to prevent inadvertent collisions between aircraft by exchanging altitude data between aircraft in close proximity. Errors in the altitude data used by this system may exacerbate rather than mitigate the risk of collision.
Aircraft systems therefore utilize various systems to verify the accuracy of the altitude data. In many cases, a redundant altitude data source/computing system is used to independently calculate the altitude information. In the event the two calculations do not agree, this information is used to indicate a system fault. Use of redundant computations, however, slows processing time and increases the overall system cost through use of duplicative circuits and components. Furthermore, this redundancy reduces but does not necessarily eliminate errors. For example, a scenario can occur where neither altimeter calculation appears to be obviously in error, yet neither is in agreement with the other.
To reduce the expense associated with a completely duplicative system, in many aircraft configurations, a single Gillham encoder is used to encode the altimetry data, or a single source of raw altimetry data is provided to the redundant computing systems for encoding. Thus, if the single encoder or single raw data source suffers a fault, even the redundant computing systems may not detect the error. A Wall Street Journal article titled xe2x80x9cFlawed Safety Device in Jets Causes a Near Miss,xe2x80x9d published Oct. 20, 1999 documents one example of this latter type of failure.
Alternative means of measuring altitude do not completely solve the above identified problems. Use of radio altimetry measures height above ground and unlike pressure altitude data or data referenced to sea level, does not provide information relative to a reference in use among all participating aircraft. Use of satellite based navigation tools such as the global positioning system, GPS, provide only a partial solution. Reliable GPS navigation requires that a minimum number of operational satellites be visible. In addition, errors in GPS computed position have their greatest magnitude in the calculation of altitude.
The present invention provides a system, apparatus, method and computer program product for enhancing the robustness of altimetry data using Gillham, or other Grey code encoding. In addition, the present invention provides a system, apparatus, method and computer program product for verifying the reasonableness of aircraft altimetry data and may optionally include an alert when the altimetry appears suspect.
According to one aspect of the invention, the invention compares two independent sources of altitude data to obtain two independent measures of altitude rate. In a preferred embodiment, the invention compares inputs from both the pneumatic altimetry data and the global positioning (GPS) altitude data. The altitude rate of change between the two sources is then compared and if the two values are not within a specified tolerance, a possible fault in the altimetry is indicated. The system faults detected by this aspect of the invention thus include a blocked static port, or xe2x80x9cstuckxe2x80x9d altitude source or encoder.
According to another aspect of the present invention, the invention verifies that the measure of pressure altitude as indicated by the altimetry system is a reasonable value. In a preferred embodiment, pressure altitude is compared with geometric altitude provided by an independent source, such as for example, GPS. An error budget is established to account for errors due to, for example, the known difference between pressure altitude and geometric altitude in the standard atmosphere, errors in GPS position measurement and deviations from standard atmospheric conditions. If the pressure altitude differs from the geometric altitude by an amount greater than this error budget, a possible fault is likely in the pressure altitude sensing/reporting system.
According to yet another embodiment of the present invention, the Gillham code is itself checked for errors. In one embodiment of the invention, the current Gillham value is compared to the last sampled Gillham value to ascertain if the Gillham bits have toggled between samples in an invalid sequence. The invention additionally checks for invalid bit values in certain code positions. The present invention is thus able to detect a fault in the Gillham logic such as a broken encoder, a stuck bit or faulty wiring connection.
According to another aspect of the invention, the invention alerts the cockpit crew of discrepancies in the altimetry data. The invention may also be used to signal other aircraft systems that the altimetry data is suspect. Systems requiring such data may then elect to disregard or otherwise process the altimetry data in accordance with the criticality of altitude data to the operation of that system.
Further features and advantages of the present invention will be described in greater detail below with reference to the drawings.